1. Technical Field
The present disclosure relates to a pre-flash time adjusting circuit, in particular, to a pre-flash time adjusting circuit which can adaptively adjust a pre-flash time of the image sensing array and image sensor thereof.
2. Description of Related Art
An optical mouse, typically includes a light source (such as a light-emitting diode) to illuminate a working surface and an image sensor that functions as a miniature digital camera to continually capture images of the illuminated working surface and to determine the speed and direction that the optical mouse is being moved across the surface by comparing sequentially captured frames of image information so as to control the movement of a cursor displayed on a monitor.
The image sensor includes an image sensing array. The image sensing array includes a plurality of pixel units for capturing image of the illuminated working surface. Please refer to FIG. 1 and FIG. 2. FIG. 1 is schematic diagram illustrating a conventional pixel unit. FIG. 2 is a timing waveform diagram illustrating the operation of the conventional pixel unit.
The pixel unit 15 includes a PMOS transistor MP1, a NMOS transistor MN1, a NMOS transistor MN2, a NMOS transistor MN3, a PMOS transistor MP2, a bipolar junction transistor (hereinafter BJT transistor) Q, a photodetector PD and a capacitor C. The PMOS transistor MP1, the NMOS transistor MN1 and the NMOS transistor MN2 compose a feedback amplifier circuit 150. The NMOS transistor MN3 and the PMOS transistor MP2 compose a shutter circuit 151. The feedback amplifier circuit 150, the shutter circuit 151 and the photodetector PD are respectively coupled to the BJT transistor Q. The capacitor C is coupled to the shutter circuit 151 and the ground. The capacitor C is configured for operatively storing the image data captured by the photodetector PD.
The operation of the pixel unit 15 can be briefly described as follow. At start of each frame, a light-emitting diode (not shown in FIG. 1) provides a light beam to illuminate the working surface, and the working surface reflects the light beam and generates a reflected light beam. The photodetector PD operatively generates a base current IB in responsive to a light intensity of the reflected light. The feedback amplifier circuit 150 begins to build up a base-emitter voltage VBE of the BJT transistor Q to increase the beta of the BJT transistor Q to a stable state e.g., ˜30. The beta herein is the ratio between the collector current IC and the base current IB of the BJT transistor Q. When the BJT transistor Q reaches the stable state, the BJT transistor Q operates with a stable beta and generates a stable emitter current IE. Next, the shutter circuit 151 is triggered on by a shutter signal such that the storage capacitor C can be discharged with a stable emitter current IE to capture the image data. Accordingly, when the shutter circuit 151 is triggered on, the voltage across the capacitor C decreases as the capacitor C discharges through BJT transistor Q with the emitter current IE.
The time required from the light-emitting diode turning on (the base-emitter voltage VBE of the BJT transistor Q starts to increase) to the time the BJT transistor Q reaching the stable state is defined as the pre-flash time of the BJT pixel circuit Q. The time required from the shutter circuit being triggered on until the completion of image capturing is defined as an exposure time. In a conventional image sensor, the pre-flash time is set to about 220 microseconds (us), and the exposure time is set to about 10 us.
However, there is a problem in the conventional image sensor. According to brightness of the image data, the pre-flash time required by the pixel unit 15 will be different. In order to achieve high image quality, the darker image data needs enough pre-flash time. In other words, the darker the image data is, the longer the pre-flash time is required. However, the brighter image does not need too long pre-flash time. Because the conventional image sensor fixes the pre-flash time, when the pixel unit 15 captures the brighter image data, the pre-flash time does not end as soon as the base-emitter voltage VBE of the BJT transistor Q reaches the stable state. The light-emitting unit will waste power if the image sensor operates with the fixed pre-flash time.